1. Field of the Invention
The present invention relates to an automatic transmission for vehicles and, more particularly to, fluid coupling power transmission such as a torque converter, which is equipped with a lockup clutch.
Description of the Prior Art
As well known in the art, the torque converter transmits a torque by feeding a helical flow off fluid establishing in a pump impeller to a turbine runner thereby to rotate It. Thus, the torque is transmitted through a fluid so that the vibration and noise caused by the torque fluctuation of an engine can be absorbed to some extent. In order to improve the transmission efficiency of power in the torque converter, on the other hand, a lockup clutch has been frequently used in the prior art. This lockup clutch directly connects a member at the input side and a member at the output side of the torque converter by mechanical means. Therefore, if the lockup clutch is engaged, the vibration to be caused by the fluctuation of the engine torque may be transmitted as it is to the automatic transmission or its output shaft to deteriorate the driving comfort.
Thus, a damper mechanism is generally used together with the lockup clutch, as disclosed in Japanese Patent Laid-Open No. 251664/1988. In a torque converter according to this disclosure, an annular weight is connected to a front cover through a damper mechanism, and a centrifugal clutch acting as a lockup clutch is arranged at the inner circumference of the annular weight and connected to a turbine runner.
In the torque converter thus constructed, as disclosed, the annular weight acts as an inertial mass for attenuating the vibration of a flywheel at the put side. At a high speed run with the centrifugal clutch being engaged, moreover, the annular weight attached through the damper mechanism acts as an inertial resistance to suppress the booming noise that is caused by a torsional vibration.
Incidentally, the lockup range, i.e., a running state with the lockup clutch being engaged is preferred to be determined depending upon a plurality of parameters such as the vehicle speed or the throttle opening, as has been generally set in the prior art. In recent years, on the other hand, in a specific running state, the lockup clutch is controlled to a slip state called the "half lockup" so as to improve the mileage and the driving comfort.
Since, however, the aforementioned centrifugal clutch functioning as the lockup clutch is engaged and released in accordance with the centrifugal force acting thereon, i t has been found different to engage without fail in a predetermined lockup range and, on the contrary, may be engaged although unnecessary.
Specifically, the centrifugal force to act upon the centrifugal clutch is changed depending upon the rotational speed of the turbine runner only. In the torque converter of the prior art described above, the lockup clutch is controlled according to only one parameter so that its control of engagement and release is difficult. Still the worse, the half lockup control at a specific running state only is almost impossible. Since, moreover, the rotational speed of the turbine runner is changed not only frequently while the vehicle is running but also seriously during a gear change, the lockup clutch may be temporarily engaged due to a temporary increase in the rotational speed of the turbine runner during the running or gear changing operation, and the accompanying change in the torque of the output shaft may appear as a shock.
On the other hand, the centrifugal clutch has to operate and establish a predetermined engaging force against the elastic force or sliding resistance of a return spring so that it is made of a massive member having a considerable weight. As a result, the torque converter of the prior art thus having the centrifugal clutch as its lockup clutch has its mass increased at the turbine runner side. Thus , the synchronization energy of the clutch at the gear changing time may be increased to deteriorate the shifting shock.
Thus, the torque converter of the prior art is made advantageous by using the annular weight in the improvement in the vibration attenuating characteristics and in the prevention of the booming noise. However, other serious disadvantages are raised by adopting the centrifugal clutch as the lockup clutch so that the torque converter of the prior art seems difficult for practical applications. It is, therefore, conceivable to replace the centrifugal clutch for causing those disadvantages by the ordinary lockup clutch of the prior art, which is arranged to face the inner surface of the front cover and actuated by oil pressure.
Since, however, the aforementioned annular clutch is supported In the so-called "floating position" by the spring of the damper mechanism, it is forced onto the inner surface of the front cover if the lockup clutch is pushed to the annular weight so as to engage the lockup clutch. As a result, the torque will be transmitted between the front cover and the annular weight by the frictional force established in between, so that the damping action of the damper mechanism is accordingly weakened. In other words, there arises a problem that a hysteresis due to the frictional resistance is increased to deteriorate the vibration attenuating characteristics.
In the torque converter described above, a coil spring is interposed between the front cover connected to the output shaft of the engine and the annular weight. Thus, there arises a problem that the inertial mass does not sufficiently function to suppress the torque fluctuation which is caused as a result of explosion of each cylinder of the engine. Specifically, the energy caused by the torque fluctuation or the rotational fluctuation is absorbed in terms of the elastic deformation of the coil spring and the increase/decrease of the number of revolution of the annular weight. The torque T which will change the number of revolution of the annular weight is expressed by: EQU T=I.multidot.d.omega./dt
(I: inertial moment; and .omega.: angular velocity). Hence, the annular weight directly acts to suppress the fluctuation of the torque or number of revolution of the engine. However, the elastic energy E of the damper spring (or coil spring) is expressed by: EQU E=k.multidot.x.sup.2 /2
(k: spring constant; and x: displacement). The damper spring does not function to suppress the fluctuation of the torque or number of revolution of the engine until it is elastically deformed. Thus, the damper spring does not directly act upon the torque fluctuation and the rotational fluctuation of the engine. Thus, the torque converter of the prior art has the damper spring (or coil spring) interposed between the front cover and the annular weight so that it either allows the torque (or rotational) fluctuation of the engine slightly or suppresses the same with a delay. As a result, in the torque converter of the prior art having the built-in annular weight, the annular weight does not sufficiently function as the flywheel for the engine but is little effective in reducing the number of idling revolution without the so-called "chattering", for example.
In case the lockup clutch is arranged in the torque converter housing, the torque transmission capacity of the lockup clutch is difficult to increase because a restriction on the space. In a device, as disclosed in Japanese Utility Model Laid-Open No. 11426/1987, for example, there is provided a piston to be actuated by oil pressure, which is formed at its leading end face with a number of recesses, so that the clutch disc may be pushed by the piston to have an enhanced engagement while being prevented by the recesses from having an abrupt increase in the engaging force.
Here, the capacity of the torque to be transmitted by the clutch is determined by parameters including the area and contact pressure of the frictional surface, the coefficient of friction and the radius of the frictional face from the center of revolution, and the aforementioned torque converter of the prior art has its torque transmission capacity increased by raising the contact pressure of the frictional face of the above-specified parameters.
However, since the torque converter has little spatial surplus therein, its size has to be enlarged if the aforementioned piston and its oil passages are to be elaborately provided. Moreover, the grooves in the leading end of the piston of the aforementioned torque converter act to change the engaging characteristics of the lockup clutch but not to increase the engaging force. Thus, the prior art has found it difficult to increase the torque transmission capacity of the lockup clutch without being accompanied by the increased size of the system.